Lactones Definition and Nomenclature of Lactones
The name of lactone is derived from a compound called as Lactide. The dehydration of lactic acid generates lactones. Lactones get their names according to the precursor acid molecule, with lactone as a suffix and a Greek letter as a prefix. For example, macrocyclic lactones are named as macrolactones. Lactones are cyclic organic esters of hydroxycarboxylic acid, usually formed when hydrogen or halogen atoms react with any carboxylic group present in the same molecule. All this process is done by intramolecular esterification of hydroxycarboxylic acid. There is a ring containing two or more carbon atoms and just one oxygen atom.
Lactones with three- or four-membered rings (α-lactones and β-lactones) are highly reactive, because of which isolation becomes difficult. Unique laboratory methods are used for both small ring lactones, and for lactones that contain rings larger than six-membered, for a reaction.
Two types of lactones occur naturally; saturated and unsaturated. Some commonly found lactones in nature are kavain, gluconolactone, neurotransmitters, ascorbic acid, and antibiotics.
Lactone Synthesis
Different methods of ester synthesis are applied to lactones. In oxandrolone synthesis, the last forming step is the organic reaction, known as esterification.
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However, in the halolactonization, halogen attacks an alkene through electrophilic addition along with the cationic intermediate. It is then captured intramolecularly by adjacent carboxylic acid.
Lactone Reactions
5-membered γ-lactones and 6-membered δ-lactones are the most stable structure because, as in every organic cycle, 5 and 6 membered rings minimize the pressure of bond angles. At room temperature, γ-lactones, along with the presence of dilute acid, are very stable. Thus, 4-hydroxy acids (R-CH(OH)-(CH2)2-COOH) instantly undergo natural esterification and cyclization to the lactone. β-lactones do prevail, but can only be made by unique methods. α-lactones can be detected as temporary species in mass spectrometry experiments.
Lactones reaction and ester reaction are very similar, and the methods applied for ester synthesis can be used here. For example, Shiina macro-lactonization, Yamaguchi esterification, and nucleophilic abstraction. Given below are some of the lactones reactions:
Hydrolysis
When a lactone with a base is heated, it hydrolyzes the lactones into its parent compound, a straight-chained bifunctional compound. The hydrolysis-condensation reaction of lactones is reversible, just like the ester reaction. After hydrolysis, lactones offer only a single product.
Reduction
By the reduction reaction, lactones get reduced to dios with the help of lithium hydride in the presence of dry ether. At first, the reaction will break the ester bond of the lactone, and later it will reduce the aldehyde group (-CHO) into the alcohol group (-OH). For example, gamma lactones get reduced to:
butane-1,4-diol, (CH2(OH)-(CH2)2-CH2(OH).
Polymerization
Lactones from polyesters, according to the below-given formula:
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Aminolysis
Lactones even react with ethanolic ammonia. First, it will break the ester bond, then will respond with the acidic -COOH group. This happens because of the fundamental properties of ammonia of forming a dual functional group, i.e., alcohol and amide. Gamma-lactones will react to produce CH2(OH)-(CH2)2-CO-NH2.
Michael Reaction
Some lactones like Sesquiterpene lactones, which are found in several plants, react with different molecules by Michael's reaction.
(Michael reaction is nucleophilic addition of a carbon or another nucleophile, with an α,β-unsaturated carbonyl compound which contains an electron-withdrawing group)
Uses of Lactones
Lactones are used for adding flavors and fragrances to fruits and dairy products. It is used for adding flavors in both fermented and unfermented dairy products. Some of the examples of lactones are;
γ-decalactone also named as 4-decanolide has a natural peach flavor
δ-decalactone also called as 5-decanolide has a creamy coconut as well as peach flavor
γ-dodecalactone also named as 4-dodecanolide has coconut and fruity flavor
γ-octalactone even named as 4-octanolide has a herbaceous character, although it's not a herb
γ-nonalactone has an intense coconut flavor of this series, but it is not present in coconut
Macrocyclic lactone has the same odor as of animal origin
Polycaprolactone is a vital plastic
Other examples of lactones are Macrolides, Ellagic acid, Kavalactones, Tergallic acid diacetone, Lactide, and Valoneic.
FAQs on Lactones
1. What exactly are lactones in organic chemistry?
In organic chemistry, a lactone is a type of cyclic ester. It is formed when a carboxyl group (-COOH) and a hydroxyl group (-OH) within the same molecule react with each other in a process called intramolecular esterification. This reaction creates a closed ring structure containing an ester linkage (-O-C=O) as part of the ring.
2. How are different types of lactones named and classified?
Lactones are classified and named using Greek letters (α, β, γ, δ, etc.) to indicate the size of the ring. The letter corresponds to the carbon atom of the original hydroxy acid that bore the -OH group which formed the ring. The counting starts from the carbon adjacent to the carbonyl group.
- α-Lactones: 3-membered rings (highly unstable).
- β-Lactones: 4-membered rings.
- γ-Lactones: 5-membered rings (e.g., γ-butyrolactone).
- δ-Lactones: 6-membered rings (e.g., δ-valerolactone).
The most common and stable lactones are the γ-lactones and δ-lactones.
3. What is the chemical process for the formation of a lactone?
Lactones are typically formed from hydroxycarboxylic acids (molecules containing both a hydroxyl and a carboxylic acid group). The formation occurs through an acid-catalysed intramolecular reaction where the hydroxyl group attacks the carbonyl carbon of the carboxylic acid group. This process eliminates a molecule of water and results in the formation of a stable cyclic ester ring. The size of the resulting lactone ring depends on the distance between the reacting functional groups.
4. What are some of the most important applications of lactones?
Lactones have diverse applications across various industries due to their distinct properties, especially their aromas. Key applications include:
- Food and Beverage Industry: Used as flavouring agents to impart fruity, creamy, or buttery notes to products like dairy, baked goods, and drinks.
- Perfumery: Valued for providing milky, fruity (like peach or coconut), and warm scents to perfumes and cosmetics.
- Pharmaceuticals: Certain complex lactones, like macrolides, function as antibiotics. Other types, such as spironolactone, are used as diuretics to treat high blood pressure.
- Polymer Chemistry: Lactones like ε-caprolactone are used as monomers to produce biodegradable polyesters like polycaprolactone (PCL), which has medical and packaging applications.
5. What is the key difference between a lactone and a lactide?
While they sound similar, a lactone and a lactide are structurally different. A lactone is a cyclic ester formed from a single molecule of a hydroxy acid. In contrast, a lactide is a cyclic di-ester formed from two molecules of a hydroxy acid (like lactic acid) reacting with each other. Essentially, a lactide contains two ester groups within its ring, whereas a standard lactone contains only one.
6. Why are gamma (γ) and delta (δ) lactones more common and stable than other types?
The stability of lactones is primarily governed by ring strain. Gamma (γ) lactones form 5-membered rings and delta (δ) lactones form 6-membered rings. These ring sizes are the most thermodynamically stable because their bond angles are very close to the ideal tetrahedral (109.5°) and trigonal planar (120°) geometries, resulting in minimal angle strain. Conversely, smaller rings like α-lactones (3-membered) and β-lactones (4-membered) are highly strained and much more reactive, making them less common and harder to synthesise.
7. How do lactones create specific scents and flavours in products like fruits and perfumes?
Lactones contribute to scents and flavours through their unique molecular structures, which interact with our olfactory (smell) and gustatory (taste) receptors. The size of the lactone ring and the length of any attached alkyl chains determine the specific aroma profile. For instance, γ-undecalactone is famous for its distinct peach aroma, while other lactones can provide creamy, coconut, or buttery notes. This makes them essential for recreating natural flavours and creating complex fragrance profiles in perfumes.
8. Are all lactones safe for consumption since they are found naturally in foods like fruits?
No, not all lactones are safe for consumption. While many naturally occurring γ- and δ-lactones are used as safe flavouring agents in the food industry in controlled amounts, the term 'lactone' covers a vast class of compounds with varied biological effects. For example, macrolide lactones are potent antibiotics. Some plant-derived lactones can be toxic. Therefore, the safety of a lactone depends entirely on its specific chemical structure and concentration, and only those approved by food safety authorities should be considered edible.
